Invalidity of propositional formula 
It may be a silly doubt but I am really confused about valid and
  invalid terms.

From whatever I read, I concluded that:
Validity and invalidity apply only to arguments, not to propositional statements.
For a (simple)statement either it can be true or false.
For arguments- It is valid if all the premises are true, then the conclusion must be true.
Invalid when it is possible that all the premises are true and the conclusion is false.
For a propositional formula (compound propositions) - a tautology, contingency, contradiction, satisfiable, unsatisfiable, valid terms make sense. 

Now my confusion is:
  
  
*
  
*Can we use the $Invalid$ term with a compound proposition (propositional formula)? If yes, Then what is the condition for its invalidity?
  

 A: It certainly makes sense to have terminology to distinguish truth/falsity of propositional statements from validity/invalidity of arguments that purport to be logical reasoning.  On the other hand it is very common for authors to use valid to describe a compound formula with assignments of truth and falsity to its atomic propositions when it results in the overall formula being true (resp. invalid when an assignment gives a false result).  
Good authors will be careful in the use of terminology (and in the definitions), but informal usage will vary.  Ultimately if you want to use the terms valid/invalid for compound propositions, you should provide consistent definitions that alert readers or listeners to your meaning.
A: Let’s first clear a terminological point.
Validity as in “this argument is valid” belongs to classic formal logic.
Validity as in “a tautology is a valid proposition” belongs to model theory.
That’s two meaning of valid. In mathematical logic we can talk about a valid proposition (in the 2nd sense of “valid”). Roughly it means true under any possible interpretation. A contradiction is the negation of a valid proposition. A contingent proposition is a neither valid nor a contradiction. Invalid means not valid, that is contradictory or contingent.
